Electronic Structures\nand Spectra of Donor–Acceptor\nConjugated Oligomers

Narrow band gap donor–acceptor conjugated polymers\npresent\nexcellent paradigms in photonics and optoelectronics due to their\nchemical tunability, correlated electronic structures, and tunable\nopen-shell electronic configurations. However, rational design for\nenhancing the properties of these molecular systems remains challenging.\nIn this study, we employed density functional theory (DFT) calculations\nto investigate prototypical narrow band gap donor–acceptor\nconjugated oligomers, consisting of alternating cyclopentadithiophene\n(CPDT) donors paired with benzothiadiazole (BT), benzoselenadiazole\n(BSe), benzobisthiadiazole (BBT), and thiadiazoloquinoxaline (TQ)\nacceptors. Analyses of structures, singlet–triplet gaps, and\nabsorption spectra of oligomers with up to ten repeat units have shown\nthat when incorporating the BT, BSe, and TQ acceptors, the backbone\ncurvature resulted in spiral structures that were energetically favored\nover their linear counterparts, causing differences in the calculated\ncircular dichroism spectra. Oligomers with BBT-based acceptors preferred,\nhowever, a linear geometry, consistent with an open-shell electronic\nstructure. Calculated singlet–triplet splittings demonstrated\nthe importance of long chains and specific structures for consistency\nwith the experiment, while effects of the solvent were also quantified.\nBased on the predicted low-energy conformations, one-photon absorption\nspectra for the considered oligomers have shown that using the Tamm–Dancoff\napproximation within time-dependent DFT for the large systems offers\ngood agreement with the first absorption maxima in measured experimental\nspectra, thus validating the method for large donor–acceptor\noligomers. Natural transition orbital analyses provided insights into\nthe excited-state characteristics. Two-photon absorption maxima were\naccurately predicted, but the cross-sections were overestimated or\nunderestimated, as dependent on the level of theory employed, to be\naddressed in future work.